For producing photovoltaic solar cells a-Si:H is a useful alternative to crystal-line silicon for reasons of economy and because of the high flexibility in production. For depositing a-Si:H on an industrial scale one mainly uses plasma enhanced chemical vapor deposition (PECVD) of silicon-containing gases or gas mixtures. One thereby deposits p-type, intrinsic, i.e. undoped, and n-type layers of amorphous silicon or amorphous silicon mixed semiconductors one on the other in this order on a transparent substrate with a transparent conductive electrode applied thereto (p-i-n cell). For depositing device-grade a-Si:H, however, the deposition rate in the PECVD method must be limited to values of about 1 .ANG./s in the large-area industrial process. Although one can in principle attain relatively high rates by high plasma excitation frequencies, high plasma powers or high silane partial pressures, in particular on small areas, the attainable deposition rate for uniformly homogeneous layers decreases as the substrate area increases. Additionally, an increased deposition rate in large-area deposition involves plasma polymerization in the vapor phase, which leads to undesirable, efficiency-reducing powder formation.
A clear increase in deposition rate has been achieved by the so-called "hot wire" (HW) method known from the literature, which deposits the intrinsic layer of a-Si:H using a heated wire which is fed silane for thermal decomposition (U.S. Pat. No. 5,397,737; J. Appl. Phys. 82(4)(1997)1909). A deposition rate of 5-10 .ANG./s is reached.
One uses a tungsten wire which is heated to a high temperature of about 2000.degree.C. during deposition of the a-Si:H layer, while the substrate on which the a-Si:H is deposited is heated to a temperature of at least 330.degree. C. and at most 630.degree. C. However, the high substrate temperature of at least 330.degree. C. is an essential disadvantage of the known method since it considerably increases the energy consumption.
In addition, solar modules with a p-i-n layer sequence on a glass/TCO substrate are already produced industrially (so-called superstrate technology). However, if the amorphous silicon is deposited on the glass/TCO substrate at a substrate temperature of more than 250.degree. C., in particular more than 300.degree. C., a clear drop in the power output of the solar cell will be noticed. This is presumably due to destruction of the p layer by diffusion of metal atoms out of the TCO layer.